Emily's Cat
Rarely prone to hissy-fits
Helium's supposed to be painless, isn't it? I think there's a place in ...Sweden?, that legally lets you off yourself, pleasantly, painlessly. It's kind of expensive, but it's all very ...well, nice. I'd seen a documentary about it one time, either on TV or on the net, I forget which. I could be mistaken, but I kind of think, I kind of half-remember, they use helium.
So whatever they use there, is probably the best way we have today to off someone. So it would seem a no-brainer to use that for death sentences, in those places that still have that barbaric punishment.
eta: Do they fill them up with drugs first, to alleviate anxiety and fear and so on? That should be common knowledge, but I'm afraid I don't know! They should, if they don't. Fill them up with a bunch of chemicals that'll have them smiling ear to ear, then hook them up to that helium contraption thing, while listening to music they like or watching some movie they like or whatever, and have them drift off quietly to sleep and death. Pretty nice way to go, that, if go you must. And no reason why it shouldn't be doable, that I can think of (well, unless people object to making it pleasant for criminals on death row).
1) I think nitrogen displaces oxygen more efficiently than helium does.
2) Nitrogen will accomplish the alleviation of fear and anxiety *while* killing them.